feat: mobile-responsive layout

- Mobile (<md): single-panel view, sidebar/chat/members as overlays
- Hamburger button toggles sidebar overlay (ServerBar + ChannelList)
- Members button toggles MemberList overlay
- ← back button in ChatArea and DMChat headers returns to sidebar
- Auto-switch to chat view when channel or DM is selected
- Tap empty area to dismiss overlays
- Desktop layout unchanged (all panels side by side)
- Status/settings buttons hidden on small screens to save space

web/src/components/DMChat.tsx

@@ -3,6 +3,7 @@ import { useParams } from "react-router-dom";
 import { useConversationStore, type ConversationMessage } from "../stores/conversation.ts";
 import { useAuthStore } from "../stores/auth.ts";
 import { useTypingStore } from "../stores/typing.ts";
+import { useLayoutStore } from "../stores/layout.ts";
 import ReactMarkdown from "react-markdown";
 import remarkGfm from "remark-gfm";
 
@@ -132,8 +133,15 @@ export function DMChat() {
 
   return (
     <div className="flex flex-col h-full bg-gb-bg">
-      <div className="terminal-border border-t-0 border-x-0 px-3 py-2 text-gb-fg-s">
-        @ {title}
+      <div className="terminal-border border-t-0 border-x-0 px-2 md:px-3 py-2 text-gb-fg-s flex items-center gap-2">
+        <button
+          type="button"
+          onClick={() => useLayoutStore.getState().setMobileView('sidebar')}
+          className="md:hidden terminal-button text-xs px-2 py-0.5 shrink-0"
+        >
+          ←
+        </button>
+        <span className="truncate">@ {title}</span>
       </div>
       <div ref={scrollContainerRef} onScroll={handleScroll} className="flex-1 overflow-y-auto p-3 space-y-1 font-mono text-sm">
         {isLoadingOlder && <p className="text-center text-gb-fg-f text-xs">[loading older messages...]</p>}